Forging stocks in simple shape were commonly used in conventional technologies. An example is disclosed in Patent Document 1 where a cylindrical shape stock is to be forged into a piston of an internal combustion engine. Another example in simple shape is disclosed in Patent Document 2. Further, another example which is shown in Non-Patent Document 1 (FIG. 6) is a cut piece of steel rod as a hot-forging stock for a crankshaft. In hot forging, the steel rod undergoes preforming, finish forming, burr trimming, and coining sequentially.